This invention relates generally to an apparatus for coupling a member implanted in a body with a member located outside of the body. More particularly the present invention is related to magnetic devices for magnetically holding and aligning the external member supercutaneously with respect to an implanted subcutaneous member.
There exist many devices in which it is desirable to impart an electrical signal to an internal location in a body. One prime example is to facilitate electrical stimulation of the auditory nerve. In these devices, an electrode is implanted in or near the cochlea of a patient and an electrical wire transmits electrical signals to the electrode which may ultimately be interpreted by the patient as representations of sound. Other devices in which it is desirable to impart an electrical signal to an internal location in the body may include heart pacers, neuromuscular stimulators and bone growth stimulators. In each of these devices it may be necessary or desirable to impart a subcutaneous electrical signal.
In order to impart electrical signals across the skin boundary, usually either a percutaneous plug is used to directly connect the wire or else an external device is located supercutaneously proximate an implanted subcutaneous device. Percutaneous plugs generally are not desirable due to the possibility of infection. When an external supercutaneous device and an implanted subcutaneous device are used, information may be transmitted electrically across the skin boundary without requiring a direct through the skin wire connection.
Where an external supercutaneous transmitter is utilized in conjunction with an implanted subcutaneous receiver, it is important to hold the external device close to the skin boundary in order to maintain a known constant physical separation between the external transmitter and the implanted receiver. It is also important to maintain accurate lateral alignment of the transmitter and receiver with respect to each other. Errors in either of these positioning constraints can cause deteriorations in the signal being received by the implanted subcutaneous receiver.
One known mechanism for holding an external device in place is with an ear hook designed to mechanically hold the external device to the external ear. Similarly, eye glass frame structures have been used to mechanically hold an external device in place. However, in both of these cases, misalignment is common due to slippage or mechanical displacement of the mechanical device from its intended position.
Another known mechanism for holding an external device in place is described in the U.S. Pat. No. 4,352,960, Dormer et al, Magnetic Transcutaneous Mount for External Device of an Associated Implant. In Dormer, a magnetic device is located in an implanted receiver and another magnetic device is located in the external transmitter. Magnetic attraction between these two magnetic devices holds the external transmitter closely against the skin and also closely in lateral alignment. Such a system provides advantages in maintaining critical spacing and alignment of an external transmitter with a subcutaneous receiver.
Because of differences in thickness of different individual skin and differences in sensitivities to pressure against the skin, differing magnetic attractive forces for the external devices worn by differing persons are desirable. If an individual's skin thickness is greater than the norm, then the spacing between the magnetic devices is greater. In order to provide the same attractive force between the supercutaneous and subcutaneous magnetic devices, a stronger magnetic device either external or implanted is required. Since the implanted magnetic device cannot be readily changed due to the surgery involved usually in order to change the magnetic force of the supercutaneous magnetic device must be modified. Also, differences in the actual magnetic attractive force are desirable. This is because the attractive magnetic force on one individual may be comfortable but the same magnetic attractive force on another individual may cause skin irritation and soreness. It is desirable to be able to vary the magnetic attractive force in order to maintain a comfort level but still adequately hold the external device in place.